Particulate matter (PM) emissions from diesel engines are regulated in the Unites States, Europe Union, Japan, China, India and other countries. Particulate matter is mainly comprised of soot, which is formed in the engine through incomplete combustion. SOF (soluable organic fraction) condensates may form on soot particles during exhaust gas cooling and dilution. Sulfates and water can also be found in diesel particulate matter.
To meet PM emissions regulations, diesel particulate filters (DPF) provide the primary approach to exhaust aftertreatment. There are two main DPF technologies, namely wall-flow DPFs and particulate oxidation catalysts (POCs). Diesel oxidation catalysts can reduce SOF condensates if designed to do so, but are not considered primary PM control devices.
Wall-flow DPFs typically provide PM reduction efficiencies greater than 90%. However, wall-flow DPFs require regular active regenerations to remove accumulated PM and thereby avoid blockage of the DPF.
POCs have PM reduction efficiencies in the range of 20% to 80%, depending on engine operating conditions, fuel quality, and the PM loading level within the POC. The objective of using a POC is not necessarily to equal the high efficiencies of a wall-flow DPF, but to achieve good PM reduction with a lessened need for active regeneration.
For POC's, to effectively reduce PM over an extended engine or vehicle operation time, e.g., over the lifetime of the engine, active regeneration may be necessary. A differential pressure sensor (ΔP sensor) is commonly used as an input to trigger POC regeneration.